Hafnium

Endpoint summary

Additional information

Hafnium is a member of group 4 of the periodic table, along with the Ti and Zr. The oxidation state is (+4). Mineralogically, hafnium is always found with zirconium and the physico-chemical properties of Hf are very similar to Zr. Both metals are passivated by a protective oxide film, which makes them almost inert towards any chemical attack (exceptions are hydrofluoric acid and alkali melts containing fluoride that dissolve both elements).

In aqueous solution, hafnium is always quadrivalent and precipitates as the Hf(OH)_{4 .} Hf(OH)₂²⁺ is the hydrated form of HfO₂, and is the only species present in solution in the absence of other stabilising ligands. This complex is very stable and resistant to protonation (Hagfeldt et al. 2004). Complexes with sulphates, fluorides and chlorides may be poorly soluble in aqueous solution, but complexation with natural organic material may increase the concentrations of Hf in natural freshwater. Studies suggest that when hafnium is detected in surface water samples it is associated with particles in the water column (Tanisaki et al. (1992); Taylor et al. (1990)).

Due to this behaviour of Hf in aquatic compartment, read across with the hydrated form HfO2 (as Hf(OH)₂² +) has been performed for several required endpoints.

In the environment, the fate of hafnium will be: 

 

Degradation:

Concerning degradation, Hf cannot be degraded in a biotic way because it is an inorganic element and as it is an elementary metal it cannot strictly undergo to hydrolysis but instead, goes through a rapid complexation with hydroxides ions (OH-) to form hydrated forms oh HfO2.

Bioaccumulation:

Hafnium compounds in water are very insoluble, and with a strong affinity with particulate organic matter showed by high Kd value,

Hf is therefore very unlikely to be bioavailable to aquatic organisms, fish and then fish-eating predators. No potential for bioaccumulation is then expected.

Adsorption:

In the terrestrial compartment, the availability of metal compounds for uptake by biota can differ from site to site and may change over time due to many processes, including weathering and (de)sorption processes. It should also be noted that Kd are accurate only during an equilibrium state, which is difficult to reach for metals in the environment. As a consequence, part of the metal present in the solid phase may be encapsulated in the mineral fraction and is therefore not available.

The IAEA Technical Reports Series No. 364 (Handbook of Parameter Values for the Prediction of Radionuclide Transfer in Temperate Environments, 1994) publication reported Kd values for Hf within the range of 1500 to 5400 L/kg, with a Kd of 2500 L/kg for all soils in general.

These values and IAEA assessor were considered reliable enough to be used for Hf, as the common metal form Hf is retrieved in soils.

The typical value kept for assessment: Kd of 2500 L/kg. This coefficient confirmed a high adsorption potential of the substance on particulate matter and low mobility in soils, leading to a poor availability to biota via interstitial water.

 

Atmospheric behaviour:

Volatilization can be ignored for metals compounds, except for mercury compounds and several organometallic compounds.